Verkruysse et al., “Remote plethysmographic imaging using ambient light”, Optics Express, 16 (26), 22 Dec. 2008, pp. 21434-21445 demonstrates that photo-plethysmography signals can be measure remotely on the human face with normal ambient light as the source and a simple digital, consumer-level photo camera in movie mode. After setting the camera in movie mode, volunteers were asked to sit, stand or lie down to minimize any movements. Color movies were saved by the camera and transferred to a personal computer. Pixel values for the red, green and blue channels were read for each movie frame, providing a set of PV(x,y,t), where x and y are horizontal and vertical positions, respectively and t is time corresponding to the frame rate. Using a graphic user interface, regions of interest (ROI) were selected in a still (selected from the movie) and the raw signal PVraw(t) was calculated as the average of all pixel values in the ROI. Fast Fourier Transforms were performed to determine the power and phase spectra. It is stated that selection of the ROI is not critical for the heart rate determination. It is also stated that limits to the spatial resolution of photoplethysmography images due to movement artifacts may be solvable by improved positioning of the volunteers, software to laterally synchronize the frames and more homogeneous illumination to reduce shading artifacts.
A problem of the known method is that it uses a supervisor to select the ROI of the part of the image he knows corresponds to the living person.
It is an object of the invention to provide a method, system and computer program of the types mentioned above in the opening paragraphs that require little or no human supervision in order to provide good results.
This object is achieved by the method according to the invention, which includes:                obtaining a sequence of digital images taken at consecutive points in time;        selecting at least one measurement zone comprising a plurality of image points; and        for each measurement zone, obtaining a signal representative of at least variations in a time-varying value of a combination of pixel values at at least a number of the image points for use in determining at least one of a presence and a frequency value of at least one peak in a spectrum of the signal corresponding to a frequency of a periodic physiological phenomenon. The step of selecting at least one measurement zone includes analyzing information based on pixel data of a plurality of image parts in at least one of the images, each image part including at least one image point, and selecting each measurement zone from contiguous parts determined to have similar characteristics.        
Analyzing information based on pixel data of a plurality of image parts in at least one of the images, where each image part includes at least one image point, can be conducted automatically, as can clustering those parts determined to have similar characteristics. Thus, this method is suitable for unsupervised execution. Selecting contiguous parts determined to have similar characteristics results in the determination of a region of the image with homogeneous characteristics. If these characteristics are similar according to an analysis in the spatial domain, a better selection of a homogeneous zone which will form the measurement zone can be made. Even if the body part corresponding to the measurement zone does not remain exactly in position throughout the sequence of images, the pixel intensities in the measurement zone will not vary appreciably due to such variations in position. This improves the quality of the spectrum of signal corresponding to the time-varying value of the combination of pixel values at at least a number of the image points, so that reliable identifications of signal peaks corresponding to heart beat or breathing rate can be made. The effect is not dependent on particular lighting conditions, making the method more robust and more suitable for remote sensing applications. By using data representative of at least part of a spectrum of a time-varying value of a combination of pixel values at at least a number of the image points, a large amount of noise can be eliminated. This allows one to use images that are obtained by capturing light reflected off a living subject. Such images can be obtained with a relatively cheap camera or sensor array. By contrast, if one were to determine the spectrum of each pixel individually and then cluster the values of the peaks, one would have to use images obtained using a very sensitive imaging device, e.g. a passive thermal imaging device.
An embodiment of the method includes performing image segmentation on at least one of the sequence of digital images to select pixel data for the analysis included in the selection step.
An effect is that the amount of pixel data that has to be analyzed in the selection step is reduced, since only certain promising ones of the segments obtained in the segmentation step need be processed.
In a variant of this embodiment, the image segmentation is performed using an algorithm for recognizing image parts corresponding to at least one type of body part of a living being.
This variant selects those image parts corresponding to parts of living beings that are suitable for an analysis to determine at least one of a presence and a frequency value of at least one peak in a spectrum of the average brightness signal corresponding to the frequency of a periodic physiological phenomenon. In principle, the method is based on the fact that the intensity of light reflected off skin varies with the frequency of periodic physiological phenomenon, i.e. the heart rate and respiration rate. Thus, a segmentation algorithm aimed at detecting skin, or body parts with shapes corresponding to those of body parts that are generally uncovered (e.g. the face of a human being) provides a pre-selection of suitable image segments, within which one or more homogeneous measurement zones are selected.
An embodiment of the method includes using a tracking algorithm to place at least one of the measurement zone and an image segment including the measurement zone in each of a plurality of the images in the sequence.
This embodiment takes account of the fact that even a homogeneous measurement zone may be affected by larger movements. The tracking algorithm allows the measurement zone to move with the actual body part it represents. Thus, signal artifacts arising from inhomogeneous image parts moving into the measurement zone are largely avoided. This improves the signal to noise ratio of the signal components corresponding to the periodic physiological phenomena.
In an embodiment, the sequence of digital images is caused to be captured by a camera upon completion of an initialization phase, the initialization phase including:                measuring periodic intensity fluctuations in at least parts of images acquired by the camera whilst camera settings are varied, and        selecting values of the camera settings at which measured periodic intensity fluctuations in at least a range of frequencies are determined to be minimal        
This embodiment allows one to remove sources of periodic disturbances, e.g. at the mains frequency. Typically, such disturbances correspond to periodic fluctuations in ambient lighting. Because the method is suitable for remote imaging, these disturbances play more of a role than would be the case if one were to use e.g. an infrared light source and camera. Intensity fluctuation measurements can be limited to one color component or be based on a weighted sum of some or all of the color components comprised in the pixel data. Suitable camera settings to be varied include the frame rate, exposure time, camera gain and pixel clock.
An embodiment of the method includes determining a correction signal corresponding to a time-varying value of a combination of pixel values at at least a number of image points in an image part other than the measurement zone, and                decorrelating at least one of the pixel data of the images in at least the measurement zone and the time-varying value of the combination of pixel values at at least a number of the image points in the measurement zone from the correction signal.        
This embodiment allows one to move non-periodic disturbances from the analysis, further improving the signal to noise ratio of the signal components due to periodic physiological phenomena. An example would be the reflections of a television signal in the face of a person watching television whilst the sequence of images is captured. It is noted that the image part other than the measurement zone may be a larger image part that also encompasses the measurement zone.
In an embodiment of the method, an output representative of whether the presence of at least one peak in the spectrum corresponding to a frequency of the periodic physiological phenomenon is detected is used to control a device arranged to perform a function conditional on detecting a presence of at least one living being of at least one kind.
An effect is that it is possible to verify that a living person is actually present in an unobtrusive way.
In a variant, the output is provided to a conditional access system for use in an authentication operation.
An effect is that it is no longer possible to fool the conditional access system that a particular person is present. For example, it is not possible to provide a fingerprint detector with a wax cast of an absent person's finger, or to fool a face recognition system with a photograph of an absent person. Thus, this method is particularly suitable for use in conjunction with a biometric conditional access system.
In an embodiment of the method, the frequency value of at least one peak in the spectrum corresponding to the frequency of the periodic physiological phenomenon is determined and a system for providing perceptible output is caused to adapt its output in dependence on the frequency signal.
Thus, this embodiment is particular suited to providing bio-feedback, e.g. in an ambient system or in a gaming or fitness environment.
An embodiment of the method includes providing a gating signal based on the signal corresponding to at least the variation in the time-varying value of the combination of pixel values at at least a number of the image points to an imaging system.
This embodiment is suitable for use with imaging systems such as MRI or CT systems, wherein a person is placed in a scanner. In such imaging systems, a gating signal is often required in order to obtain a still image of e.g. a heart. An unobtrusive heart or respiration rate determination is preferred to lower the stress level of the person being imaged. Furthermore, there are no wires or probes that might affect the imaging system.
An embodiment of the method includes:                selecting a plurality of measurement zones;        for each measurement zone, obtaining a signal representative of at least variations in a time-varying value of the combination of pixel values at at least a number of the image points and determining the frequency value of at least one peak in a spectrum of the signal corresponding to a frequency of a periodic physiological phenomenon; and        detecting how many living beings are represented in the sequence of images by comparing the frequency values.        
This method is suitable for an image segmentation method for example. It allows one to discern between different people in the images. Other applications include image analysis systems for crowd control, for example.
According to another aspect, the system for processing images of at least one living being according to the invention includes:                an interface for obtaining data representative of a sequence of digital images taken at consecutive points in time; and        an image data processing system, configured to:        select at least one measurement zone comprising a plurality of image points; and        for each measurement zone, to obtain a signal representative of at least variations in a time-varying value of the combination of pixel values at at least a number of the image points for use in determining at least one of a presence and a frequency value of at least one peak in a spectrum of the signal corresponding to a frequency of a periodic physiological phenomenon, wherein the image data processing system is configured to select the at least one measurement zone by analyzing information based on pixel data of a plurality of image parts in at least one of the images, each image part including at least one image point, and to select each measurement zone from contiguous parts determined to have similar characteristics.        
In an embodiment, the system is configured to carry out a method according to the invention.
According to another aspect of the invention, there is provided a computer programme including a set of instructions capable, when incorporated in a machine-readable medium, of causing a system having information processing capabilities to perform a method according to the invention.